Roman society was one that constantly pushed romans to be more and more ambitious, to take more, do more and conquer more. Eventually you start stepping on people's toes who are trying to do the same thing, then you have two powerful people fighting for ultimate power (ceaser v. pompey, sulla v. marius, augustus v. marc anthony, etc.). Then there was the Marian reforms which made soldiers beholdened primarily to their general, not the state, for their rewards (usually land after the campaign was finished), couple that with legions frequently going further and further from Rome in the late republic, most Roman soldiers knew and depended on their general, and barely interacted with the state at all. So these generals gradually gained ferociously loyal armies that were closer to them than Rome in general, so they'd be pretty willing to fight for their general against another general, even when it would weaken the state as a whole. Obviously civil wars cause a huge amount of damage to their nation, both in lives and monetary cost. Plus usually whoever won the civil war would then proceed to kill all prominent citizens who even slightly leaned toward the opposing side. After two or three purges like this, many of the prominent families that made rome into a world power were completely in shambles and the bitter rivalries between them made future wars inevitable.
